Nine people are feared to have died when a US coastguard Hercules plane collided with a Marine Corps helicopter off the coast of southern California.

The crash was reported at 7.10pm local time yesterday. The Federal Aviation Administration (FAA) said a pilot had reported seeing a fireball about 50 miles off the coast of San Diego county near San Clemente, the southernmost of a chain of islands, which is owned by the US navy and used for training. Officials said the cause of the crash was unknown.

A Marine Corps spokesman said an AH-1W Super Cobra helicopter was on a training mission when it went down. The helicopter had been based at Camp Pendleton, a training facility on the southern Californian coast.

The coastguard plane belonged to a unit that flies search-and-rescue, law enforcement and logistics missions. Captain Thomas Farris, of the coastguard, said the plane was searching for a person reported to have attempted to row a dinghy to another island in the channel.

"The assumption is always that they are alive," Farris said. "We have not found any human remains at this point, or survivors." He estimated that water temperatures would permit survivors to last up to 20 hours.

Ian Gregor, an FAA spokesman, said two people were on board the helicopter and seven on the C-130 Hercules, a four-engined turboprop plane used around the world as a troops and cargo carrier.

He said coastguard crews had seen debris from the plane. Three coastguard and US navy ships and helicopters were scouring the area. Weather conditions at the time of the collision were reported to have been good and remained clear for the search.

This week a Cobra was involved in a collision with another US helicopter in Afghanistan, killing four US troops. In May, two marines died when their Super Cobra crashed in a forest in southern California. A subsequent investigation found a transmission cover that had not been properly secured flew off and hit the tail rotor.